As a semiconductor device becomes finer, an exposure wavelength used in a semiconductor exposure apparatus becomes shorter down to a KrF laser (with a wavelength of 248 nm), an ArF laser (with a wavelength of 193 nm), and a F2 laser with a wavelength of 157 nm. At the same time, the NA of a projection optical system becomes higher up to 0.90 in an ordinary atmosphere, and beyond 1.2 for a projection optical system of an immersion type exposure apparatus.
The fine patterning of a semiconductor device is a major factor that supports the dynamics of the semiconductor industry, and the generation is swiftly changing from the age that required a resolution of 0.25 μm at 256 MDRAM to 180 nm, 130 nm and further to 100 nm and beyond. In the age of the lithography using the i-line (365 nm), they didn't realize the resolution less than the wavelength. However, the KrF, having a wavelength of 248 nm, has been applied to the critical dimension of 180 nm down to 130 nm. The age that practically uses the resolution less than the wavelength has really arrived through the progress of resist and the use of results out of resolution enhancement technologies (RET), etc. Various RET would reduce the critical dimension to one-third of the wavelength in line and space patterns.
However, RET often has pattern-induced limits, and therefore, the royal road to the improvement of resolution is, after all, to make the wavelength short while improving the NA of a projection optical system. Recently, a minute imaging analysis emphasizes considerations of parameters that have been ignorable in the past, such as flare, and the polarization due to the property of the light as electromagnetic waves.
Among them, the issue of polarization has gradually had a great impact as a projection optical system's NA becomes larger. The issue that polarization presents is such that when two rays intersect each other, they don't interfere with each other if the two rays' polarized directions are orthogonal to each other. If two rays are symmetrically arranged to the optical axis, the angle of the optical axis with one ray becomes 45°. The NA close to 0.71 causes a pair of rays to satisfy this orthogonal condition. Therefore, a current projection optical system having more than 0.80 already met the condition in which the imaging rays do not interfere with each other in the aerial image.
The effect of this orthogonal condition becomes more prominent in an immersion type exposure apparatus, because even if the orthogonal condition is present in the aerial image obtained in the air, nitrogen, or helium circumstances (hereinafter called dry), an angle θPR that the light entering a resist having a refractive index of nPR at an angle θo has in the resist is expressed as follows:sin θo=nPR sin θPR  (1)The angle θPR thus becomes smaller than θo, and does not satisfy the orthogonal condition in the resist.
Usually, since the refractive index of the resist at a wavelength of 193 nm is about 1.7. If θPR becomes 45°, the right-hand side of the equation (1) becomes 1.7×sin 45°=1.20, which is more than 1. Therefore, θPR 45° condition never exists in the dry case.
However, in the immersion exposure that fills with liquid the space between a resist and a projection optical system, the refraction effect is greatly reduced, and θPR can be 45°.
Some solutions for this issue have been proposed which control the polarization of an illumination optical system and maintain the contrast of an image formed by a projection optical system. (See, for example, Japanese Patent Applications, Publication Nos. 8-008177, 4-366841, 5-088356, 5-090128,6-124872, 6-181167, and 6-188169.)
In order to expose a pattern with a high resolution, RET takes a measure to control an angular distribution for illuminating reticles and to construct an optimal illumination optical system. Various proposed light source shapes, such as quadruple, dipole, sextuple as well as the conventional simple annulus contribute to enlarge the exposure latitude and the depth of focus. A CGH (Computer Generated Hologram) inserted as a diffraction optical element into an illumination optical system provides flexibility to form various light source shape requirements, thus making an important contribution to the progress of optical lithography. See, for example, Japanese Patent Applications, Publication Nos. 2001-284212 and 11-176721.
However, the control of polarization that has become especially conspicuous for an immersion type exposure apparatus, and the flexibility of the illumination optical system are newly required issues.
An illumination optical system should be suitable for a high NA optical system, such as a projection optical system used in an immersion type exposure apparatus, while reconciling polarization to the optical system having a diffraction optical element.